rtl-sdr.com

Hello guy's! Since im from germany, my english is not the best but i hope you can help me with my problem!

First of all, this isn't the first double cross antenna i've build. The first one was a one day holiday project, meant to last only a few days, so no expensive materials were used. I even forgot about the velocity factor in the coax line

Surprisingly this antenna worked pretty good, and i received some meteor images. Since i didn't use an LNA, the signal was very weak below 20 degrees altitude.

----

Now, one year later, i've build a new one out of pvc pipes to withstand the weather, everything sealed waterproof. I even bought a noaa saw filtered lna to get a stronger signal. Powered over bias with a nooelec smartee

After some first test i was pretty impressed, even below 10 degrees i was able to decode a picture using Sdr#, Meteor Demodulator and DDE plugin. I received pictures of greenland, iceland, sweden, russia, england and also part of africa

Now comes the strange part: as the satellite is raising over the horizon, the signal gets strong pretty quick, apart from some radiation nulls caused by our roof. As soon as it hits >80 degrees, the signal gets weak (even pulsing a bit), resaulting in lost lines across the image. Seems like there's a huge radiation null


--- After doing some research i noticed that i wired all four dipoles upside down, meaning the center conductor is connected to the lower part.

--- Since i made this mistake with all four, it shouldn't be big of a deal?.. or is it?

Second problem is another radiation null located at south, maybe caused by our satellite dish? I'm able to get pictures of iceland, which is pretty far north, but africa is nearly impossible. Would like to fix that too


Here are some details about the antenna:

4mm2 blank cooper wire, cut to length (545mm each, two per dipole, so every dipole is 1090mm long)
RG58 coax (with a velocity factor of 0,66) cut to two pieces of 400mm, and two to 760mm, so 1/4 longer
Each pair of dipoles is spaced apart exactly 1/4 of a wavelength!
Pvc pipes do not contain any metal materials (to add strength for example)


Well that's it for now, i hope some kind person could help me with that problem! If you need more details or pictures, feel free to request them

P.s.: North is marked on the antenna by the red velcro!

EDIT: After climbing on the roof a second time, to turn all four dipoles around (center cond. up), nothings changed. Still same inconsistent signal when receiving meteor.

Does anyone have a clue about what is going on? Many people have been reading this thread since it was uploaded, but nobody wants to help?

Sorry it's just not easy to troubleshoot antenna issues.

How does the signal look like when you're receiving it?

Also what exactly is the Nooelec LNA that you have? AFAIK they haven't actually released a 137 MHz LNA yet. Are you sure that the LNA you have is for 137 MHz? Got a link to the one you have?

rtlsdrblog wrote: ↑

Sun Apr 14, 2019 8:48 am

Sorry it's just not easy to troubleshoot antenna issues.

How does the signal look like when you're receiving it?

Also what exactly is the Nooelec LNA that you have? AFAIK they haven't actually released a 137 MHz LNA yet. Are you sure that the LNA you have is for 137 MHz? Got a link to the one you have?

Hello rtlsdrblog, thank for the answer, i appreciate that!

So the LNA i bought is not made by Nooelec, it's by a british company called Uputronics. They work together with Airspy i think. The LNA is placed directly at the antenna, only about two feet of coax away.

The signal becomes visible on the spectrum pretty early, but rather than consistently getting stronger, it keeps varying in strength. As soon as it reaches the 90 degrees elevation, it nearly disappears.

What is strange to me is the fact that the antenna works great in the northern region, but fails miserably at south? When i took the picture of iceland, the satellite was more than 2600 kilometers away..

Our house is about ten meters high, so height can't be an issue? Or maybe the signal get's reflected by the roof and cancels out itself?

I would expect this antenna to have a null directly above and below.

https://www.amsat.org/articles/w6shp/lindy.html

HighSNR wrote: ↑

Sun Apr 14, 2019 1:33 pm

I would expect this antenna to have a null directly above and below.

https://www.amsat.org/articles/w6shp/lindy.html

Thanks for the link. So a double cross dipole is known to have a null at 90 degrees? But why does everybody else using a double cross get a strong signal directly over head? Even my first DCA with wrong coax phasing, no LNA and placed one meter above the ground was able to receive a consistent signal.

Its rare for a satellite to actually be directly overhead. Most commonly they will acutally be at a lower elevation.

The Lindenblad (double cross) is designed to give circular polarisation as a replacement for a linear dipole.

For coverage at 90degree elevation you can use a turnstyle(crossed dipole) antenna or a helix or quadrafiler helix.

HighSNR wrote: ↑

Sun Apr 14, 2019 2:13 pm

Its rare for a satellite to actually be directly overhead. Most commonly they will acutally be at a lower elevation.

The Lindenblad (double cross) is designed to give circular polarisation as a replacement for a linear dipole.

For coverage at 90degree elevation you can use a turnstyle(crossed dipole) antenna or a helix or quadrafiler helix.

Okay, so the only way for a full omnidirectional reception would be a antenna like the QFH? I always thought the DCA does exactly that

What about the last image i posted? The antenna was placed close to the ground, could this have caused some kind of ground reflection that helped to overcome the 90 degree null?

swarl3s wrote: ↑

Sun Apr 14, 2019 11:49 am

So the LNA i bought is not made by Nooelec, it's by a british company called Uputronics. They work together with Airspy i think. The LNA is placed directly at the antenna, only about two feet of coax away.

Ah okay, I know about the Uputronics LNA. This one has the LNA before the SAW filter (so the signal into the LNA is not filtered), so i'd suggest checking the spectrum to see if there are any artifacts or signs of the LNA overloading. In particular broadcast FM is nearby in frequency and that could overload the LNA. That might reduce your sensitivity enough to make the nulls in the antenna become an issue. Might explain why your previous try without the LNA worked better. So try without the LNA too just to rule it out.

If it's not that then I can only think that it is something to do with the satellite dish and roof geometry causing greater nulls. Possible to reposition it and try again?

ANtenna performance can vary significantly if the dimensions (diameter and lengthof radiator,coax type length) of the reference design are not copied. Also a faulty soldering joint or connection can make the difference betwen works and does not work. Did you use this double cross antenna design https://www.qsl.net/l/lu7did/docs/NOAA/ ... MARTES.pdf ? T
The antenna elevation pattern is also site specific, meaning ground reflections (elevation and strength of notches in the pattern) and obstacles arround the antenna that can shaed direction in those directions.
As for RFI it can be a single strong signal or a combination of multiple strong signals that genearte intermodulation . This can be not just strong TV or broadcast stations, but also other VHF transmitter closeby e.g. aeronautical 108 MHz -137 MHz or HAM 144 -146 in D.
If the antenna and LNA are ok, you might also check for local interference in the band 137 -138 in or around your house like CATV leackage, oscillation TV antenna with builtin preamp, powerline...... .You name it and most like such an interference has been identified in Germany at one location or another.